In recent years, ring resonators (“resonators”) have increasingly been employed as an essential component in optical networks and other nanophotonic systems that are integrated with electronic devices. A resonator can ideally be configured with a resonance wavelength substantially matching a particular wavelength of light. When the resonator is positioned adjacent to a waveguide within the evanescent field of light propagating along the waveguide, the resonator evanescently couples the particular wavelength of light from the waveguide and traps the light for a period of time. Resonators are well-suited for use in modulators and detectors in nanophotonic systems employing wavelength division multiplexing (“WDM”). These systems transmit and receive data encoded in different wavelengths of light that can be simultaneously carried by a single optical fiber or waveguide. Resonators can be positioned at appropriate points along the optical fiber or waveguide and operated to encode information by modulating unmodulated wavelengths of light and detect wavelengths of light encoding information and translate the encoded wavelengths into electrical signals for processing.
A resonator's dimensions directly affect the resonator's resonance wavelength, which is particularly important because in typical WDM systems the wavelengths may be separated by fractions of a nanometer. However, even with today's microscale fabrication technology, fabricating resonators with the dimensional precision needed to insure that the resonator's resonance wavelength matches a particular wavelength of light can be difficult. This problem arises because the resonance wavelength of a resonator is inversely related to the resonator's size. In other words, the resonance wavelength of a small resonator is more sensitive to variations in resonator size than that of a relatively larger resonator. For example, a deviation of just 10 nm in the radius of a nominally 10 μm radius resonator results in a resonance wavelength deviation of 1.55 nm from the nominal resonance wavelength for which the ring resonator was designed. This 0.1% deviation approaches the limits in accuracy for fabricating resonators using optical lithography. A deviation of this magnitude is undesirable and in fact may be unacceptable in typical optical networks and microscale optical devices where the wavelength spacing may be less than 1 nm.
In addition to inaccuracies encountered during fabrication of resonators, environmental conditions under which the resonators are operated can change the resonance wavelength of the resonators. In particular, a temperature change in the resonator shifts the effective refractive index of the resonator resulting in deviations from a desired resonance wavelength. This is problematic in integrated CMOS-nanophotonic systems where the power dissipation and temperature of adjacent circuitry varies substantially over time.
Thus, it is desirable to tune transmitting and receiving resonators in order to compensate for resonance wavelength deviations that may be due to environmental conditions and fabrication inaccuracies.